The capacity of the immune system to prevent the expansion of autoreactive B and T cells is a critical step in the prevention of autoimmunity, and T cell anergy may plan a role in maintenance of peripheral tolerance. The binding of B7 (expressed on APC), to the T cell costimulatory molecule CD28, provides a signal that is required for T cell activation and proliferation. T cell anergy is induced in the absence of the CD28 mediated signal. Manipulation of costimulatory pathways may provide new approaches to treating autoimmune disease and cancer, however, little is known about the costimulatory requirements of CD28- T cells which account for half of human CD8+, CD8+, CD4-8- alphabetaTCR+, and gamma delta TCR+ T cells. To understand the pathways of activation used by CD28-T cells, we have examined the costimulatory requirements of antigen specific human CD4-CD8- alphabetaTCR+T cell lines that lack the expression of CD28. DN1 and DN6 are mycobacteria specific CD1+ monocytes and CD1+ B lymphoblastoid cells (B-LCL) proliferation occurred only when antigen was presented by the CD1+ monocytes. This suggested that the CD1+ monocytes expressed a costimulatory molecule that the B-LCL transfectants lacked. This hypothesis was confirmed by showing that anergy was induced when the T cells were incubated with antigen pulsed CD1+ B-LCL. The required signal occurs via a CD28-independent mechanism and represents a previously unrecognized pathway of costimulation for T cells. This grant proposes to use human T cell lines that require a non-CD28 costimulatory signal to identify and characterize the molecules belonging to this second costimulatory pathway. In Aim 1, costimulation of CD28- T cells will be studied, focusing on the induction of anergy. Antigen specific CD28 CD8+ T cell lines will be derived to ascertain whether CD28- T cells other than DN1 and DN6 require the costimulatory signal provided by CD1+ monocytes. In Aim II, the T cell costimulatory molecule will be identified and characterized. This will involve the production of specific antibodies, functional, biochemical and molecular analysis including cloning the cDNA that encodes the molecule, and determining the tissue distribution of the antigen. In Aim III, the costimulatory ligand expressed by CD1+ monocytes will be identified and characterized as above. This application is for a Clinical Investigator Award to an MD PhD who has completed postgraduate training in internal medicine and rheumatology. These studies are sponsored by Dr. Michael Brenner in the Department of Rheumatology at Brigham and Women's Hospital and Harvard Medical School. The applicant's long term goal is to establish and direct his own independent research program.